I'm all for decreasing the collection times, like the current level upgrades do, but it would be nice to see increased output as well. I hate to use the example of Dragon Story (I really don't like that game), but they have small farms, large farms, and some 3rd level. The current Stable/Hatchery buildings are 2x2, maybe a new group of levels would make this a 3x3 with much increased output, at least 2x since we've more than doubled the square footage.
So, Leveling from 5->6 might increase the building size, 2x the output, and require 3 cows/chickens (just the basic ones you buy for coins), and some combination of beams/blocks/etc.
Level up from 6->7 might increase output another 1.5x, require more cows/chickens, etc.